1. Field of the Invention
This invention relates to a method for imparting resistance to axial displacement of convolutions in a convoluted substrate of the type comprising convolutions of superimposed plain and corrugated strips of metal wound around a common axis to form a generally cylindrical substrate having end faces transverse to the axis of the cylindrical substrate wherein convolutions of plain metal strip are spaced apart by convolutions of corrugated metal strip to define a plurality of passageway channels extending axially between the end faces of the substrate. Such substrates when coated with a layer of refractory metal oxide are suitable for use in supporting catalyst in a catalyst unit of the type used in the purification of exhaust gases which are passed under pressure through the axially extending passageway channels to the substrate. The substrates are especially suitable for use in catalyst units used in the purification of exhausts from motor vehicles although they may also be used in purifying exhausts from industrial plants or as catalyst supports in general.
2. Description of the Prior Art
U.S. Pat. No. 4,186,172 (to Scholz) discloses a cylindrical convoluted structure for use in exhaust purification. The structure comprises spirally wound superimposed strips of plain and corrugated thin catalytic metal foil which co-operate to define axially extending passageways through which exhaust gases for purification are passed. The outermost surfaces of the foils are welded thereby preventing unwinding of the strips. U.S. Pat No. 4,186,172 does not teach the application of a layer of refractory metal oxide to its catalyst metal foils presumably because this would inhibit their catalytic activity. It has been found that if convoluted structures similar to those of U.S. Pat. No. 4,186,172 are used as a substrate for a catalyst carried on a layer of refractory metal oxide, then axial displacement forces are generated when exhaust gas is passed through the substrate. These forces cause an axial displacement of the central convolutions relative to one another and eventually the end face of the substrate assumes an approximately parabolic shape.
U.S. Pat. No. 4,282,186 (to Nonnenmann et al) discloses cylindrical structures for use in exhaust purification and which are similar to those of U.S. Pat. No. 4,186,172 except that the cylindrical structures of U.S. Pat. No. 4,282,186 optionally may have cross-sections which are non-circular. U.S. Pat. No. 4,282,186 is concerned with the failure of the prior art to solve the problem of axial displacement of the convolutions when exhaust gases are passed through the structures and accordingly it teaches that the plain and corrugated strips should be welded together at all their points of contiguity. Such a technique is effective but expensive because in practice the commercially useful structures comprise hundreds of points of contiguity per square inch of the end face of the structure.